A Diac is two terminal , three layer bi directional device which can be switched from its off state for either polarity of applied voltage.
Construction:
The diac can be constructed in either npn or pnp form.The two leads are connected to p-regions of silicon separated by an n region. the structure of diac is similar to that of a transistor differences are
There is no terminal attached to the base layer
The three regions are nearly identical in size. the doping concentrations are identical to give the device symmetrical properties.
When a positive or negative voltage is applied across the terminals of Diac only a small leakage current Ibo will flow through the device as the applied voltage is increased , the leakage current will continue to flow until the voltage reaches breakover voltage Vbo at this point avalanche breakdown of the reverse biased junction occurs and the device exhibits negative resistance i.e current through the device increases with the decreasing values of applied voltage the voltage across the device then drops to breakback voltage Vw
V- I CHARECTERISTICS OF A DIAC
For applied positive voltage less than + Vbo and Negative voltage less than -Vbo , a small leakage current flows thrugh the device. Under such conditions the diac blocks flow of current and behaves as an open circuit. the voltage +Vbo and -Vbo are the breakdown voltages and usually have range of 30 to 50 volts.
When the positive or negative applied voltage is equal to or greater than tha breakdown voltage Diac begins to conduct and voltage drop across it becomes a few volts conduction then continues until the device current drops below its holding current breakover voltage and holding current values are identical for the forward and reverse regions of operation.
Diacs are used for triggering of triacs in adjustable phase control of a c mains power. Applications are light dimming heat control universal motor speed control
Source: http://www.srmuniv.ac.in/downloads/diac.doc
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